The coopers Kit consists on one tin of Malt extract to which you add an additional kilo of fermentables (Either Sugar, Brew Enhancer or dry malt extract). For the stout kit, to make the best of it, I have found that a kilo of Muntons medium Spraymalt gives the best results (Spray malt comes in several grades from Extra Light through to dark) . . . Logic would say that for a stout you would want to use the darkest possible, but I think the results are less than palatable.
One other thing to be aware of when using malt extract is that the finishing gravity will be higher than stated in the instructions (probably around 1.012-1.015 rather than 1.006), as malt extract is not as fermentable as sugar.